The anchor (<a> tag) is an inline tag. Heading tags (<h1>, <h2>, ..., <h6>) are all block-level tags. It is invalid code to place a block-level tag within an inline tag. To resolve the issue you will need to either change from a heading tag to a span tag for that text (and style it to make it resemble the heading) or else break up that link so that one instance only contains the image and the other instance is inside of the heading tag.